Why is your outer skin basically brand new every month?
The skin you are touching right now did not exist a month ago.
The epidermis, your skin's outer layer, constantly rebuilds itself. New cells are born at the bottom, push upward over a few weeks, die into a tough protective layer, then flake off, so the whole outer layer turns over in roughly a month in young adults (and slower as you age).
That fine dust on your phone screen and bedsheets is partly old you - dead skin cells you shed without feeling a thing.
Your outer skin is a slow conveyor belt: made at the base, shed at the top, fully refreshed in about a month.
It is why scrapes heal and why moisturizer or sunscreen pays off over weeks, not minutes - you are treating skin that keeps replacing itself.
New skin rides up like an escalator - born at the bottom, stepping off the top in about 30 days.
Learn the idea and practice English at the same time.